Add lesson about set operation queries

4 weeks ago · 2cdeff349a
parent 5b1fd1e0b8
commit 2cdeff349a
2 changed files with 245 additions and 14 deletions
--- a/src/data/courses/sql-mastery/chapters/multi-table-queries/lessons/combining-query-results.md
+++ b/src/data/courses/sql-mastery/chapters/multi-table-queries/lessons/combining-query-results.md
@ -0,0 +1,245 @@
+---
+title: Combining Query Results
+description: Learn how to use UNION, INTERSECT, and EXCEPT to combine results from multiple queries
+order: 150
+type: lesson-challenge
+setup: |
+  ```sql
+  CREATE TABLE fiction_books (
+      id INT PRIMARY KEY,
+      title VARCHAR(255),
+      author VARCHAR(255),
+      price DECIMAL(10, 2)
+  );
+
+  CREATE TABLE non_fiction_books (
+      id INT PRIMARY KEY,
+      title VARCHAR(255),
+      author VARCHAR(255),
+      price DECIMAL(10, 2)
+  );
+
+  INSERT INTO fiction_books (id, title, author, price)
+  VALUES 
+      (1, 'The Great Gatsby', 'F. Scott Fitzgerald', 24.99),
+      (2, '1984', 'George Orwell', 19.99),
+      (3, 'Pride and Prejudice', 'Jane Austen', 15.99),
+      (4, 'The Hobbit', 'J.R.R. Tolkien', 29.99),
+      (5, 'The Doors of Perception', 'Aldous Huxley', 12.99);
+
+  INSERT INTO non_fiction_books (id, title, author, price)
+  VALUES 
+      (1, 'A Brief History of Time', 'Stephen Hawking', 29.99),
+      (2, 'The Art of War', 'Sun Tzu', 19.99),
+      (3, 'Sapiens', 'Yuval Noah Harari', 24.99),
+      (4, 'Pride and Prejudice: A Study Guide', 'John Smith', 12.99),
+      (5, 'The Doors of Perception', 'Aldous Huxley', 12.99);
+
+  CREATE TABLE summer_reads (
+      id INT PRIMARY KEY,
+      title VARCHAR(255),
+      author VARCHAR(255),
+      price DECIMAL(10, 2)
+  );
+
+  INSERT INTO summer_reads (id, title, author, price)
+  VALUES 
+      (1, 'The Great Gatsby', 'F. Scott Fitzgerald', 24.99),
+      (2, 'Sapiens', 'Yuval Noah Harari', 24.99),
+      (3, 'The Art of War', 'Sun Tzu', 19.99);
+  ```
+---
+
+Sometimes you need to combine results from multiple queries into a single result set. SQL provides several set operations for this purpose: `UNION`, `UNION ALL`, `INTERSECT`, and `EXCEPT`.
+
+## UNION and UNION ALL
+
+`UNION` combines results from two or more queries and removes duplicates. `UNION ALL` does the same but keeps duplicates.
+
+I have setup the following two tables `fiction_books` and `non_fiction_books` with the following data:
+
+| id  | title                   | author              | price |
+| --- | ----------------------- | ------------------- | ----- |
+| 1   | The Great Gatsby        | F. Scott Fitzgerald | 24.99 |
+| 2   | 1984                    | George Orwell       | 19.99 |
+| 3   | Pride and Prejudice     | Jane Austen         | 15.99 |
+| 4   | The Hobbit              | J.R.R. Tolkien      | 29.99 |
+| 5   | The Doors of Perception | Aldous Huxley       | 12.99 |
+
+| id  | title                              | author            | price |
+| --- | ---------------------------------- | ----------------- | ----- |
+| 1   | A Brief History of Time            | Stephen Hawking   | 29.99 |
+| 2   | The Art of War                     | Sun Tzu           | 19.99 |
+| 3   | Sapiens                            | Yuval Noah Harari | 24.99 |
+| 4   | Pride and Prejudice: A Study Guide | John Smith        | 12.99 |
+| 5   | The Doors of Perception            | Aldous Huxley     | 12.99 |
+
+### UNION Example
+
+Let's find all books across fiction and non-fiction categories:
+
+```sql
+SELECT title, author, price
+FROM fiction_books
+UNION
+SELECT title, author, price
+FROM non_fiction_books
+ORDER BY title;
+```
+
+The query above will combine the results from both the queries and remove any duplicates. Output will be:
+
+| title                              | author              | price |
+| ---------------------------------- | ------------------- | ----- |
+| 1984                               | George Orwell       | 19.99 |
+| A Brief History of Time            | Stephen Hawking     | 29.99 |
+| Pride and Prejudice                | Jane Austen         | 15.99 |
+| Pride and Prejudice: A Study Guide | John Smith          | 12.99 |
+| Sapiens                            | Yuval Noah Harari   | 24.99 |
+| The Art of War                     | Sun Tzu             | 19.99 |
+| The Doors of Perception            | Aldous Huxley       | 12.99 |
+| The Great Gatsby                   | F. Scott Fitzgerald | 24.99 |
+| The Hobbit                         | J.R.R. Tolkien      | 29.99 |
+
+Notice how the `The Doors of Perception` book is only listed once even though it appears in both the `fiction_books` and `non_fiction_books` tables.
+
+### UNION ALL Example
+
+If you want to keep duplicates, use UNION ALL:
+
+```sql
+SELECT title, author, price FROM fiction_books
+UNION ALL
+SELECT title, author, price FROM non_fiction_books
+ORDER BY title;
+```
+
+The result will be:
+
+| title                              | author              | price |
+| ---------------------------------- | ------------------- | ----- |
+| 1984                               | George Orwell       | 19.99 |
+| A Brief History of Time            | Stephen Hawking     | 29.99 |
+| Pride and Prejudice                | Jane Austen         | 15.99 |
+| Pride and Prejudice: A Study Guide | John Smith          | 12.99 |
+| Sapiens                            | Yuval Noah Harari   | 24.99 |
+| The Art of War                     | Sun Tzu             | 19.99 |
+| The Doors of Perception            | Aldous Huxley       | 12.99 |
+| The Doors of Perception            | Aldous Huxley       | 12.99 |
+| The Great Gatsby                   | F. Scott Fitzgerald | 24.99 |
+| The Hobbit                         | J.R.R. Tolkien      | 29.99 |
+
+Notice how the `The Doors of Perception` book is listed twice.
+
+## INTERSECT
+
+`INTERSECT` returns only the rows that appear in both result sets. Let's find books that are in both the `fiction_books` and `non_fiction_books` tables:
+
+```sql
+SELECT title, author FROM fiction_books
+INTERSECT
+SELECT title, author FROM non_fiction_books;
+```
+
+This will return:
+
+| title                   | author        |
+| ----------------------- | ------------- |
+| The Doors of Perception | Aldous Huxley |
+
+## EXCEPT
+
+`EXCEPT` returns rows from the first query that don't appear in the second query. Let's find the books in the `fiction_books` table that aren't in the `non_fiction_books` table:
+
+```sql
+SELECT title, author FROM fiction_books
+EXCEPT
+SELECT title, author FROM non_fiction_books
+ORDER BY title;
+```
+
+This will return:
+
+| title               | author              |
+| ------------------- | ------------------- |
+| 1984                | George Orwell       |
+| Pride and Prejudice | Jane Austen         |
+| The Great Gatsby    | F. Scott Fitzgerald |
+| The Hobbit          | J.R.R. Tolkien      |
+
+---
+
+## Requirements
+
+The key requirement for all of these set operation queries listed above is that the columns in both queries must have the same number, order, and data types.
+
+For example, below is an invalid query:
+
+```sql
+-- ERROR: each UNION query must have the same number of columns
+SELECT title, author FROM fiction_books
+UNION
+SELECT title, author, price FROM non_fiction_books;
+```
+
+Because the `fiction_books` table has 2 columns and the `non_fiction_books` table has 3 columns, the query will return an error.
+
+Similary if we try to combine the following two queries:
+
+```sql
+-- ERROR: UNION types numeric and character varying cannot be matched
+SELECT title, price FROM fiction_books
+UNION
+SELECT title, author FROM non_fiction_books;
+```
+
+This will also return an error because the `price` column from the first query doesn't have the same data type as the `author` column from the second query.
+
+## Challenge Time! 🎯
+
+Using the provided tables, write queries to:
+
+1. Find all books that cost exactly $19.99 across all categories
+2. List authors who have written both fiction and non-fiction books
+3. Find summer reads that aren't in our fiction collection
+
+<details>
+<summary>Solution</summary>
+
+```sql
+-- 1. Books that cost $19.99
+SELECT title, author, 'Fiction' as category
+FROM fiction_books
+WHERE price = 19.99
+UNION
+SELECT title, author, 'Non-Fiction' as category
+FROM non_fiction_books
+WHERE price = 19.99;
+
+-- 2. Authors in both categories
+SELECT author FROM fiction_books
+INTERSECT
+SELECT author FROM non_fiction_books;
+
+-- 3. Summer reads not in fiction
+SELECT title, author FROM summer_reads
+EXCEPT
+SELECT title, author FROM fiction_books;
+```
+
+</details>
+
+## Key Takeaways
+
+- Use UNION to combine results and remove duplicates
+- Use UNION ALL when you want to keep duplicates (it's faster than UNION)
+- Use INTERSECT to find common rows between results
+- Use EXCEPT to find rows in one result that aren't in another
+- Column count, order, and data types must match across queries
+- These operations are particularly useful when working with similar but separate data sets
+
+Remember: Choose the appropriate set operation based on your needs:
+
+- UNION/UNION ALL for combining results
+- INTERSECT for finding common elements
+- EXCEPT for finding differences
--- a/src/data/courses/sql-mastery/chapters/multi-table-queries/lessons/self-joins-and-usecases.md
+++ b/src/data/courses/sql-mastery/chapters/multi-table-queries/lessons/self-joins-and-usecases.md
@ -18,20 +18,6 @@ setup: |
         (4, 'Emily Davis', 2),        -- Reports to Sarah
         (5, 'James Wilson', 2),       -- Reports to Sarah
         (6, 'Lisa Anderson', 3);      -- Reports to Michael
-
-  CREATE TABLE product (
-      id INT PRIMARY KEY,
-      name VARCHAR(255),
-      substitute_product_id INT
-  );
-
-  INSERT INTO product (id, name, substitute_product_id)
-  VALUES (1, 'iPhone 13', 2),
-         (2, 'iPhone 14', 3),
-         (3, 'iPhone 15', NULL),
-         (4, 'Galaxy S21', 5),
-         (5, 'Galaxy S22', 6),
-         (6, 'Galaxy S23', NULL);
  ```
 ---